Sodium Plays Role in Certain Turfgrass Processes

Sodium (Na) constitutes about2.8percent of the earth's crust, in similar proportion to potassium (K) at 2.6 percent. Both elements are chemically similar and exist in soils as monovalent cations (they have a single positive charge). As such, they are bound to cation exchange sites on soil colloids. However, positive sodium ions (Na) attract a larger shell of hydration water than does positive potassium ions (K). This thicker hydration shell partially insulates the positive charge of Na, making its electrostatic attraction to negative cation exchange sites slightly weaker than that of K. Thus, while both ions have a concentration of. 1 millimoles (mM) to 1 mM in the soil solution of temperate region soils, Na is more readily leached to the subsoil by percolating rain or irrigation water. In arid and semi-arid regions where leaching occurs less frequently, irrigated soils often contain 50 mM to 100 mM Na (often as salt, (NaCl). Such salt concentrations are toxic to many crop plants, including most turfgrasses. Because high salt concentrations are common in soils of regions with limited rainfall or subject to tidal flooding, some plants have evolved mechanisms for tolerating excessive levels of Na. These plants are classified as natrophilic species or halophytes and are characterized by absorbing large amounts of Na and negative chlorine ions (Cl), transporting them through the xylem to leaves. Then they are sequestered in the vacuoles of mesophyll and parenchyma cells. The high salt content of leaves lowers the cell's water potential and establishes a water-potential gradient through the xylem to the roots, where water has a higher potential. Thus, water can be absorbed from saline or drying soils and transported to the leaves following the water-potential gradient. Such plants tend to have the succulent leaves characteristic of many dry-land or coastal marsh plants. Some halophytic plants can excrete excess salts to their leaf surfaces through salt glands. This feature is common to tidal marsh plants that grow in seawater and lose excreted salt to high tide floodwaters twice each day. Upland plants growing in saline soils can also have salt glands and discharge excess salts through them. Several warm-season turfgrasses possess salt glands, including bermudagrass, zoysiagrass and buffalograss. In a recent study, K.B. Marcum et al. (2003) at Arizona State University in Phoenix reported that the density of salt glands on leaf surfaces of 15 zoysiagrass cultivars correlated positively with clipping production and turf quality when grown under high-salinity conditions. This confirms the long-held belief that salt-gland excretion contributes significantly to salinity tolerance in halophytic grasses. Since salt-gland density is a genetically controlled characteristic, these authors concluded that measuring salt-gland density should be a simple way of screening turfgrass genotypes for salt tolerance. By comparison, most plants are intolerant of high Na concentrations, and survive in the presence of elevated salt levels through root exclusion, efflux and sequestration of Na. These plants are known as natrophobic species or glycophytes. The K transport channels in their root cells discriminate against Na often by a ratio of 25 to 1 or more. Still, some Na will enter the cells in amounts that are potentially toxic. To eliminate this problem, Na efflux pumps in root cell plasma membranes excrete Na out of the cells back into the cell walls in exchange for positive hydrogen ions (H) that enter the cell. Root cells normally transport Hs across their plasma membrane into the cell wall (apoplasm), where the pH is lowered by two units below that of the cytoplasm. These H7Na antiporters (H7Na exchangers) can keep the cytoplasmic Na levels to physiologically acceptable concentrations. If the H7Na antiporters can't keep up with Na influx, cortical and xylem parenchyma cells will accumulate the excess Na in their vacuoles. The excess positive charges will be balanced by Clions entering from the soil or by organic acid anions